Field
The present invention relates to communication systems. More particularly, the present invention relates to the field of wireless or mobile telecommunication networks. Even more particularly, the present invention relates to high speed transmission networks, such as wireless telecommunication networks belonging to the so called fourth generation (4G), including networks based upon Long Term Evolution (LTE) and/or Long Term Evolution Advanced (LTE-A) standards.
Overview of the Related Art
High-speed transmission networks, or simply networks, are able to simultaneously provide different services (e.g., voice and/or video real-time communications, data transfer, Web browsing, broadcasting, etc.) to a given number of user equipments (UEs, such as mobile phones, personal digital assistants, tablets, personal computers etc.).
Networks generally include a plurality of base stations, each of which manages communications for a given number of UEs. In the following, without any limitation, the term “eNode B” (eNB) (which is specifically used in LTE/LTE-A system) will be used as synonym of “base station”. In general, one eNode B may control one or more cells.
Generally, networks comprise cells of various size and are therefore identified as a heterogeneous networks, or HetNets. Particularly, each cell may be categorized as a macro-cell, a pico-cell, or other types of cell according to the size of the covered geographic area. A macro-cell is a relatively large geographic area (e.g., an area having a radius in the order of the kilometers such as one or more city blocks) and the associated eNB—which is usually denoted as MeNB—allows unrestricted access to UEs therein. A pico-cell is a relatively small geographic area (e.g., with a radius in the order of hundred of meters such as a large building) and the associated eNB—which is usually denoted as PeNB—may both allow a restricted or unrestricted access to UEs.
Furthermore, in a HetNet, the MeNB are deployed in a regular way forming a substantially continuous overall coverage area for the network; conversely, PeNB and/or other types of eNB are deployed in a quite random fashion. Therefore, very often occurs that inside a macro-cell one or more pico-cells and/or other types of cells are deployed. In this way, one or more pico-cells and/or other cell types result superimposed with a macro-cell.
In operation, each UE establishes a communication with a cell via downlink and uplink channels for accessing the abovementioned services (i.e., the UE is connected to the cell). The downlink refers to the communication link from the eNB to the UE, and the uplink refers to the communication link from the UE to the eNB.
In order to achieve the required high transmission speeds, networks utilizes orthogonal frequency division multiplexing (OFDM) for downlink communications. Conversely, single-carrier frequency division multiplexing (SC-FDM) is used for uplink communications since the high Peak-to-Average Power Ratio (PAPR) property of OFDM makes the same less favorable for uplink communication. OFDM and SC-FDM partition the system bandwidth into multiple orthogonal sub-carriers. The spacing between adjacent sub-carriers may be fixed, and the total number of sub-carriers may be dependent on the system bandwidth. The system bandwidth may also be partitioned into sub-bands, where one sub-band is formed by a certain number of adjacent sub-carriers.
Nonetheless, a downlink communication may experience interference due to concurrent transmissions performed by neighboring cells. Conversely, an uplink communication may cause interference to concurrent transmissions performed by other UEs communicating with the neighboring cells. Such interferences degrade performance on both the downlink and uplink.
Networks usually implement frequency diversity techniques for communicating. In case of wideband communication systems (such as in LTE and LTE-A systems), frequency diversity allows a signal to be spread over the frequency domain, resulting in a higher resistance to frequency selective fading, natural interference and noise. For example, SC-FDMA may spread information through all the available sub-carriers, so in case of loss of partial information on one (or even more) sub-carriers does not necessarily lead to lose the information modulated in the communication.
Unfortunately, each different type of cell usually determines a corresponding transmission power level for the downlink, which may exacerbate interference issues between neighboring cells. In detail, macro-cells usually impose the highest downlink transmit power (e.g., 20 W) in the wireless telecommunication network to the connected UEs, granting each downlink communication to reach any UE in any point in the whole macro-cell. Conversely, pico-cells, and/or other types of small cells impose a lower downlink transmission power (e.g., down to 1 W), since their coverage areas are smaller than the ones of the macro-cells. Consequently, downlink communications with low transmission power may suffer severe interferences from having downlink communications with higher transmission power.
Similarly, the interference problem also arises in the uplink as a consequence of the different coverage areas of macro and pico-cells, and is exacerbated by the fact that a power control system may increases an uplink transmission power trying to overcome such interference potentially provoking further interferences between uplink transmissions of the same cell, which may lead to severe interfering scenarios. For example, considering a macro-cell enclosing a pico-cell, the uplink communications between UE connected to the pico-cell and the pico-cell itself are likely to suffer severe interferences from uplink transmission performed by UE connected to the macro-cell close to the pico-cell.
Moreover, power control procedures provided in the uplink for adjusting the transmission power levels of UEs cannot be directly used to avoid such interference problems. Indeed, the power control system of a cell is effective only on UEs connected to the same cell while any further UEs connected to another cell and causing interferences cannot be power controlled by the cell that is victim of the interference.
In the art, solutions have been proposed in order to reduce the interference arising in HetNet, devising methods for controlling the transmission power of the eNB and of UE connected to such eNB as disclosed in the paper 3GPP R3-121299, “Analysis of Solutions for Mitigation of UL Interference in CB-ICIC”. TSG-RAN WG3 #76 Prague, Czech Republic, 21-25 May 2012.
Moreover the International patent application No. WO 2011/150296 discloses methods and an apparatus for uplink radio link monitoring in a Long Term Evolution system with enhanced inter-cell interference coordination. Various options are presented in an effort to transmit a sounding reference signal of a UE device served by an eNB in the HetNet, avoiding both interference from uplink transmissions from other UE being served by neighboring eNBs and collisions with the UE own channel quality information or physical uplink shared channel.
Furthermore, the International patent application No. WO 2012/048174 discloses systems and methods for managing inter-cell interference coordination actions for time-domain partitioned cells. In certain aspects, time-domain partitioning is accounted for by an eNB in determining whether to send frequency-based inter-cell interference information (e.g., uplink overload indicator) to neighboring eNB(s) and/or responsive actions to take in response for receiving frequency-based inter-cell interference information (e.g., uplink overload indicator, high interference indicator, and/or relative narrowband transmission power).
Finally, the International patent application No. WO 2012/024454 discloses an apparatus and a method for controlling inter-cell interference comprising detecting and measuring uplink interference; and reporting the level of the uplink interference to an inter cell interference coordination server using a backhaul link. The apparatus or method may include receiving a measured uplink interference level through a first backhaul link, determining a transmit power level based on the measured uplink interference level, and sending through a second backhaul link the transmit power level for reconfiguring either a UE or a Femto eNode B.